[MAJOR] make the client side use stream_sock_process_data()

[MAJOR] make the client side use stream_sock_process_data()

The client side now relies on stream_sock_process_data(). One
part has not yet been re-implemented, it concerns the calls
to produce_content().

process_session() has been adjusted to correctly check for
changing bits in order not to call useless functions too many
times.

It already appears that stream_sock_process_data() should be
split so that the timeout computations are only performed at
the exit of process_session().